Systems and methods for lithography masks

ABSTRACT

Structure of mask blanks and masks, and methods of making masks are disclosed. The new mask blank and mask comprise a tripe etching stop layer to prevent damages to the quartz substrate when the process goes through etching steps three times. The triple etching stop layer may comprise a first sub-layer of tantalum containing nitrogen (TaN), a second sub-layer of tantalum containing oxygen (TaO), and a third sub-layer of TaN. Alternatively, the triple etching stop layer may comprise a first sub-layer of SiON material, a second sub-layer of TaO material, and a third sub-layer of SiON material. Another alternative may be one layer of low etching rate Mo x Si y ON z  material which can prevent damages to the quartz substrate when the process goes through etching steps three times. The island mask is defined on the mask blank by using various optical proximity correction rules.

BACKGROUND

In semiconductor processing, multiple lithography steps are generallyused to form a semiconductor chip. These steps typically include forminga photoresist over a substrate, exposing the photoresist to a pattern oflight generally controlled by a mask, developing the pattern in thephotoresist to expose the underlying substrate, and etching the patternin the underlying substrate. The pattern etched in the underlyingsubstrate may be a basis for some feature formation, such as an ionimpurity implantation such as for doping source and drain regions, aformation of a structure like a gate pattern, or a pattern for aconductive material such as in a metallization layer.

Photoresists may be classified into two groups: positive resists andnegative resists. A positive resist is a type of photoresist in whichthe portion of the photoresist that is exposed to light becomes solubleto the photoresist developer. The portion of the photoresist that isunexposed remains insoluble to the photoresist developer. A negativeresist is a type of photoresist in which the portion of the photoresistthat is exposed to light becomes insoluble to the photoresist developer.The unexposed portion of the photoresist is dissolved by the photoresistdeveloper.

Advances in semiconductor processing have generally allowed forcontinued reduction of minimum feature sizes for semiconductor chips,which have increased the requirement on the image resolution of the maskused in lithography. If a mask is not able to precisely form a patternin a photoresist, the subsequently formed feature may not meet itscritical dimension requirement.

For advanced lithography imaging, the improvement of resolution is onetough challenge. Conventional positive tone imaging (PTI) technology hasreached its limit and cannot get any better image resolution with enoughprocess window. The negative tone imaging (NTI) technology using abright mask has the potential to provide a better resolution than thePTI technology using a dark mask. However, current NTI technology usinga bright mask suffers serious substrate loss during the developingprocess. Therefore there is a need to develop a better bright mask usedin the NTI technology for advanced lithography imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1( a) illustrates an exemplary mask blank, and FIG. 1( b)illustrates an exemplary island mask formed on the mask blank shown inFIG. 1( a);

FIGS. 2( a)-2(k) illustrate an exemplary process of making the islandmask shown in FIG. 1( b) from the mask blank shown in FIG. 1( a); and

FIGS. 3( a) and 3(b) illustrate the optical proximity correction (OPC)rules used in forming the island shown in FIG. 1( b).

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the disclosedsubject matter, and do not limit the scope of the different embodiments.

Embodiments will be described with respect to specific contexts, namelythe structure of mask blanks and masks, and the methods of making masks.Through newly designed mask blank structures, new pattern sizing rules,and new process flows, lower quartz damage can be achieved for negativetone imaging (NTI) bright masks. The new mask blank and the new maskcomprise a tripe etching stop layer which can prevent damages to thequartz substrate when the process goes through etching steps threetimes. The triple etching stop layer may comprise a first sub-layer oftantalum containing nitrogen (TaN), a second sub-layer of tantalumcontaining oxygen (TaO), and a third sub-layer of TaN. Alternatively,the triple etching stop layer may comprise a first sub-layer of SiONmaterial, a second sub-layer of TaO material, and a third sub-layer ofSiON material. Another alternative for the triple etching stop layer maybe one layer of low etching rate Mo_(x)Si_(y)ON_(z) material which canprevent damages to the quartz substrate when the process goes throughetching steps three times. The island mask is defined on the mask blankby using various optical proximity correction (OPC) rules.

FIG. 1( a) illustrates an exemplary mask blank, and FIG. 1( b)illustrates an exemplary island mask formed on the mask blank shown inFIG. 1( a). A mask may be referred as a photo mask, or a photomask aswell. The mask blank 100 illustrated in FIG. 1( a) comprises a substrate101, a triple etching stop layer 102 made of three sub-layers 1021,1022, 1023 on the substrate 101, a shielding layer 103 on the tripleetching stop layer 102, a hard mask layer 105 on the shielding layer103, and a positive chemical amplified resist (PCAR) layer 107 on thehard mask layer 105. FIG. 1( b) illustrates an island mask 200 formedwith the mask blank 100, wherein the island is formed on a substrate201, comprising a triple etching stop layer 202 on the substrate made ofthree sub-layers 2021, 2022, and 2023, and a shielding layer 203.

Photomasks used for lithography contain the pattern of the integratedcircuits. In a lithographic processing, integrated circuits aremanufactured by exposing a pattern of features that are contained on amask or a photomask onto a wafer. Light passing through the transparentportions of the mask activates light sensitive resist materials on thewafer that are then chemically and mechanically processed to create thecircuit features. The basis of a mask is called a blank, or a maskblank. The manufacturing of photomasks on mask blanks is basically equalto the wafer fabrication. The difference is the exposure of the resistwhich is done by electron beams (photomasks) or with optical lithography(wafer).

There are many kinds of photomasks. An attenuated phase shift mask (APSMor AttPSM, also known as a half tone mask) may use a patterned layer ofmolybdenum silicide (MoSi) which represents the structures of thecircuit. The molybdenum silicide layer has a thickness which causes aphase shift of the transmitted light of 180°. The mask 200 shown in FIG.1( b) may be an APSM or a super binary mask made of more than two layersof materials.

The substrate 101 shown in FIG. 1( a) may be made of quartz or CaF2,which is optically transparent with respect to incident light. Thesubstrate 101 may comprise a transparent substrate with a reflectivematerial coated on a surface. The substrate 101 may be an industrystandard thickness. The substrate 101 may be a variety of shapesincluding squares, circles, ovals, rectangles, etc. The substrate 101may be a low thermal expansion material (LTEM), such as, ultra lowexpansion (ULE) glass manufactured by Corning.

A triple etching stop layer made of three sub-layers 1021, 1022, and1023 may be formed on the substrate 101, which may be collectivelyreferred as a triple etching stop layer 102. It may be referred as abarrier layer 102 as well. The triple etching stop layer 102 may be usedas the stop layer for performing etching on the mask blank three timeswithout damaging the substrate, which will be illustrated in FIGS. 2(a)-2(k). An exemplary triple etching stop layer 102 may comprise a firstsub-layer of tantalum containing nitrogen (TaN) 1021 around a thicknessof about 2˜5 nm, a second sub-layer of tantalum containing oxygen (TaO)1022 around a thickness of about 2˜3 nm formed on the first sub-layer,and a third sub-layer of TaN 1023 around a thickness of about 2˜5 nmformed on the second sub-layer. Such an exemplary triple etching stoplayer may be called an NON etching stop layer due to the materials used.Some other materials may also be used, such as tantalum (Ta) alone,tantalum containing nitrogen and oxygen (TaON), tantalum containingboron (TaB), a composite material of them, or the like. Variousstructures such as TaO, Ta₂O₃, TaO₂, and Ta₂O₅ may exist.

The three sub-layers 1021, 1022, and 1023 may be of the same ordifferent thickness, with a low reflectivity of about less than <15%, anextinction coefficient k ranged about 0.55˜0.65, and a refractive indexn ranged about 2.4˜2.6. The triple etching stop layer 102 may be made bysputtering or physical vapor deposition (PVD) process or any relatedprocess.

Alternatively, the triple etching stop layer 102 may comprise a Nitride(SiON) for the first sub-layer 1021, a second sub-layer 1022 of TaO onthe first sub-layer, and a third sub-layer of SiON on the secondsub-layer. Another option for the triple etching stop layer 102 is touse one layer of any extremely lower etching rate Mo_(x)Si_(y)ON_(z)material instead of the three sub-layers 1021, 1022, 1023 as shown inFIG. 1( a), to be used as a stop layer for performing etching on themask blank three times without damaging the substrate.

A shielding layer 103 may be formed on the triple etching stop layer102. The shielding layer 103 may have a single-layer structure or aplural-layer structure. The shielding layer 103 may comprise molybdenumsilicide oxynitride (MoSiON) material, or molybdenum silicon material.The shielding layer 103 may have a thickness around a range of 550˜700°A, with an optical density (OD) in a range about greater than 3.0. Theshielding layer 103 may have a light transmissivity from about 4% to10%. When molybdenum silicon oxynitride is used, it has a lighttransmissivity of about 6%. The shielding layer 103 may be made bysputtering or PVD process.

The shielding layer 103 may be replaced by a half-tone layer or ahalf-tone phase shift layer 103 formed on the triple etching stop layer102. The half-tone phase shift layer 103 or the half-tone layer 103 isable to produce a 180 Â° phase shift on the light passing through. Thehalf-tone layer 103 may comprise a material selected from, for example,chromium oxy-nitride (CrON), chromium oxide (CrO), molybdenum siliconoxy-nitride (Mo_(x)Si_(y)ON_(z)), amorphous carbon or silicon nitride(SiN). The half-tone layer 103 may have a thickness around a range of550˜700° A, with an optical density in a range greater than 3.0. Thehalf-tone layer 103 may have a light transmissivity about a range from4% to 10%. The half-tone layer 103 may be made by sputtering or PVDprocess.

A hard mask layer 105 may be formed on the shielding layer 103 or thehalf tone layer 103. The hard mask layer may be made of chrome. The hardmask layer 105 may be made by sputtering process or PVD process. Thehard mask 105 may have a thickness in a range of about 50˜100° A.

A positive chemically amplified resist (PCAR) layer may be coated ontothe surface of the hard mask 105, with 800˜1500° A resist coating. Theuse of chemical amplification is to increase the sensitivity to theexposure energy in order to combat the larger absorption at shorterwavelengths. A PCAR may be a commercially available product, such asSEBP-9012 or SEC-9012, SEC-9093, from Shin-Etsu Chemical Co., Ltd.

The mask blank shown in FIG. 1( a) can be used to form an island maskshown in FIG. 1( b). The structure shown in FIG. 1( b) comprises asubstrate 201 which is equivalent to the substrate 101 of FIG. 1( a), anisland on the substrate, formed by a triple etching stop layer 202 and ashielding layer 203. The triple etching stop layer 202 on the substratecomprising three sub-layers 2021, 2022, and 2023, which are part of thetriple etching stop layer 102 after the rest of the sub-layers have beenetched away in the process. A shielding layer or a half-tone layer 203on the triple etching stop layer 202 is part of the shielding layer orthe half-tone layer 103 shown in FIG. 1( a), after the rest of the layer103 has been etched away in the process.

FIGS. 2( a)-2(k) illustrate an exemplary process of making the islandmask shown in FIG. 1( b) from the mask blank shown in FIG. 1( a). FIG.2( a) is the same mask blank shown in FIG. 1( a), and FIG. 2( k) is thesame island mask shown in FIG. 1( b).

In the process shown in FIGS. 2( a)-2(k), dry etching will be performed,in various etching mode such as the endpoint plasma etching mode, thetime mode, or the over etching mode. The dry etching may be performedusing a fluorine-based gas such as SF6, CF4, C2F6, or CHF3 in dryetching of a tantalum-based material. The dry etching may be performedusing a chlorine-based gas such as Cl2 or CH2Cl2, He, H2, N2, Ar, C2H4,or the like. The dry etching may also be performed using a mixed gas ofa fluorine-based gas and a chlorine-based gas, or a mixture of chlorineand oxygen gas.

As illustrated in FIG. 2( b), an island or contact island A of the PCARlayer 107 has a size equal to the size of the final island mask 203shown in FIG. 2( k), in addition to any tool bias correction needed. Thesize here may refer to the width at the cross-section view. It may referto the diameter or other measurement when needed in other view. Theisland A is surrounded by a ring trench B on both sides.

In order to develop the final mask, a plurality of optical proximitycorrection (OPC) rules is used. The addition of tool bias correction inthe size is such an OPC rule. Due to wave optics (e.g. diffraction)there can be aberration during the exposure of photomasks. Thus the OPCrules have been introduced in semiconductor manufacturing, which caneliminate or reduce image defects. OPC means to modify the structures onthe mask in such a way that the shape of the image on the masks lookslike desired. Furthermore there can be additional structures just forminimizing aberration which do not have any function for the integrateddevice itself.

The OPC rules applied in the step at FIG. 2( b) are shown in moredetails in FIGS. 3( a) and 3(b). The size of the island A is the finalisland size in addition to any tool bias correction needed, where thetool bias correction size may be in a range about 6˜20 nm. In order todevelop an island A of FIG. 2( b) surrounded by a ring trench B, atwo-step process shown in FIG. 3( a) may be followed.

In the first step shown in FIG. 3( a)(ii), a larger shape B shown as 303surrounding the shape of the island A shown as 301 is first defined. Theshape B may be an intermediate shape. The expansion of the shape of theisland A to the shape B is defined by either Rule 1 or Rule 2. The shapeB may be obtained by Rule 1, where the shape of the island A is expandedin all sides by 2 um. The shape B may be obtained by Rule 2, where theshape of the island A is expanded on all sides by half a size of thecritical dimension (CD), which may be less than 2 um. The choice ofeither Rule 1 or Rule 2 depends on the density of device layouts. Asshown in FIG. 3( b), the expansions of the different shapes of 301 and401 are kept as much disjoint as possible. But it can be overlapped ifneeded.

In the second step shown in FIG. 3( a)(iii), a “NOT A” operation isperformed on the B shape. The NOT A operation takes out the shape of theisland A from the intermediate B shape, so that the remaining part of Bis along the edges of the shape of the island A, and surrounding theshape of island A on all sides like a ring structure, as shown in FIG.3( a)(iii). The remaining part of B is used to generate the firstexposure area, which is along the edges of the island A. A cross-sectionview of the result is shown in FIG. 2( b), where the remaining part of Bdefines a ring trench B lies on both sides of A.

In summary, the size of the island A may be defined by an OPC rule: size(A) is equal to a final island mask size plus a tooling bias, where thetooling bias is equal to the first and the second exposure alignmentoverlap error and bias in the current exemplary process flow. If anyadditional exposure is used, then the tooling bias would take intoconsideration of such bias as well. The shape B is defined by a secondOPC rule: shape (B) is obtained by expanding all side of the shape ofisland A by an expansion size, where the expansion size is about 1˜2 umfor a more isolated pattern, and the expansion size is about ½*(smallestspace critical dimension) for a dense device layout. The third OPC ruleis to Perform (B) NOT (A) rule to generate the first exposure area asshown in FIG. 2( b). The exposure area will be along the edges of theisland A like a ring around the island A.

As illustrated in FIG. 2( b), the PCAR layer 107 has a ring trench Bsurrounding the island A. The island A of the PCAR layer 107 has a sizeas the size of the final island mask 203 shown in FIG. 2( k), inaddition to any tool bias correction needed. The ring trench B is formedby the shape shown in FIG. 3( a)(iii).

As illustrated in FIG. 2( c), the hard mask layer etching is performedto expand the ring trench B along both sides of A, to the hard masklayer 105. The etching may be done using chemical Cl2/O2 as an example.

As illustrated in FIG. 2( d), the shielding layer or the half-tone layeretching may be performed to expand the ring trench B along both sides ofA, to the shielding layer or the half-tone layer 103. The etching may bedone using chemical SF6/O2 as an example.

As illustrated in FIG. 2( e), the PCAR layer 107 is completely stripped.As the result, a well-defined contact island A with ring trench B is nowmade. By this stage, the remaining process behavior will almost be thesame as PTI-dark mask making. A good critical dimension control of thecontact island can be achieved without being impacted by any loadingdifference.

As illustrated in FIG. 2( f), a second round photoresist layer 109 maybe coated onto the surface of the remaining pattern to protect thecontact island A. The second round photoresist may be a negativechemically amplified resist (NCAR) or a PCAR. Examples of an NCAR may bea commercially available product, such as SEC-2014 from Shin-EtsuChemical Co., Ltd. The NCAR layer 109 fills the ring trench B andfurther covers the surface of the hard mask layer 105. The NCAR layer109 may be with 800˜1500° A thickness on the surface of the hard masklayer 105.

As illustrated in FIG. 2( g), the NCAR layer 109 is exposed with ane-beam or an optical tool on the part to be kept. Developing process canthen be performed so that only a part of the NCAR layer 109 remains onthe contact island A. The developing process can be done by using adiluted alkaline developing solution or a deionized water (DIW) solutioncontaining about 0.05 to 0.37 weight percent tetramethylammoniumhydroxide (TMAH). Full puddle developing method may be used with staticand dynamic swing 60˜120 sec. The total remaining NCAR layer 109 may beover 50% in the ring trench B area to afford the etching loading effect.

As illustrated in FIG. 2( h), an etching of the hard mask layer 105 maybe performed to remove the hard mask layer 105 not covered by thephotoresist layer 109, which is separated from the island A. The etchingmay be an endpoint mode plasma etching, done with Cl2/O2 at 200/10 sccm,under 4 mT pressure. Through this etching step, the top sub-layer 1023of the triple etching stop layer 102 is also etched away. The plasmaetching also performs lateral etching so that the sub-layer 1023 underthe NCAR 109 is etched away as well.

As illustrated in FIG. 2( i), an etching of shield layer or half-tonelayer 103 may be performed to remove the shield layer or half-tone layer103 not covered by the photoresist layer 109, which is separated fromthe island A. The etching may be an endpoint mode plasma etching, donewith SF6/O2=5/60 sccm, under 4 mT pressure. Through this etchingprocess, the second sub-layer 1022 of the triple etching stop layer 102may also be etched away. The plasma etching also performs lateraletching so that the sub-layer 1022 under the NCAR 109 is etched away aswell.

As illustrated in FIG. 2( j), the remaining NCAR layer 109 is completedremoved, so that the hard mask layer 105 and the shielding layer 103over the island A is not covered by NCAR layer 109 anymore. In theillustration, a NCAR layer is used. The procedure would operatesimilarly if a PCAR layer is used as the photoresist layer 109 at stepshown in FIG. 2( f).

Finally, as illustrated in FIG. 2( k), an etching of the hard mask layer105 may be performed to remove the hard mask layer 105 remaining on topof the shielding layer 103 at the island. The etching condition can bethe same as previous etching of the hard mask layer 105 shown in FIG. 2(h). Through this etching process, the third sub-layer 1021 of the tripleetching stop layer 102 may also be etched away. As a result, a finalisland mask is formed shown in FIG. 2( k). It is an NTI-bright mask withgood critical dimension quality control and without quartz damage isproduced.

Although the present embodiments and their advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims. Moreover, the scope of the present application is not intendedto be limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A mask blank, comprising: a substrate; a tripleetching stop layer on the substrate, wherein the triple etching stoplayer comprises a first sub-layer of SiON material, a second sub-layerof TaO material on the first sub-layer, and a third sub-layer of SiONmaterial on the second sub-layer; a shielding layer on the tripleetching stop layer; and a hard mask layer on the shielding layer.
 2. Themask blank of claim 1, further comprising a positive chemical amplifiedresist layer on the hard mask layer.
 3. The mask blank of claim 1,wherein the first sub-layer, the sub-layer, and the third sub-layer ofthe triple etching stop layer are with a reflectivity less than about15%, an extinction coefficient k ranged about 0.55˜0.65, and arefractive index n ranged about 2.4˜2.6.
 4. The mask blank of claim 1,wherein the shielding layer comprises a molybdenum silicide oxynitride(Mo_(x)Si_(y)ON_(z)) material or a molybdenum silicon material.
 5. Themask blank of claim 1, wherein the shielding layer is replaced by ahalf-tone layer.
 6. The mask blank of claim 5, wherein the half-tonelayer comprises a material selected from a group consisting essentiallyof chromium oxy-nitride (CrON), chromium oxide (CrO), molybdenum siliconoxy-nitride (Mo_(x)Si_(y)ON_(z)), amorphous carbon, or silicon nitride(SiN).
 7. The mask blank of claim 1, wherein the substrate is a quartzsubstrate.
 8. The mask blank of claim 1, wherein the hard mask layercomprises chrome.
 9. A mask comprising: a substrate; an island on thesubstrate, having a triple etching stop layer on the substrate, and ashielding layer on the triple etching stop layer, wherein the tripleetching stop layer of the island comprises a first sub-layer of SiONmaterial, a second sub-layer of TaO material on the first sub-layer, anda third sub-layer of SiON material on the second sub-layer.